Previous experiments have found an asymmetry in the results from feature positive and feature negative discrimination procedures. A successive discrimination develops readily with the feature positive procedure but fails to develop in most cases with the feature negative procedure. Previous research by the author investigated the effects of varying the temporal arrangement of stimulus features within the compound stimulus trials of these procedures. In the feature negative case, the use of sequential compound stimuli reliably produced differential responding to the S and S minus trials, whereas the use of simultaneous compound stimuli resulted in no discrimination between trials. The proposed experiments will investigate this difference obtained in the feature negative discrimination performance obtained with simultaneous and sequential compounds. The first experiment will determine whether the differential effectiveness of the two procedures in producing the successive discrimination is due to a difference in learning about the B feature and nonreinforcement or to difference in the ability of the B feature to suppress responding to the A feature. The second experiment will investigate the development of conditioned inhibition by the B feature in the simultaneous feature negative and sequential feature negative procedures using both a summation and an acquisition test. The third experiment will examine several variations of the feature negative procedure to isolate the critical aspect of the sequential feature negative procedure which is o necessary for the successive discrimination to develop. These experiments will provide information about the mechanisms involved in the production or absence of the discrimination and will contribute to an understanding of the process of discrimination in a differential reinforcement procedure.